PANDA Straw Tube Tracker (STT)

General Description

* General view of Straw Tube Tracker:

A straw tube is basically a single channel drift tube, i.e. a gas filled tube with a wire along its axis. A high voltage of some kV is applied between the wire and the tube so that an electric field is present in the gas filled area. When a charged particle transverses the straw tube, ionization takes place; the electrons drift toward the wire, while the ions drift toward the wall. Once the electrons are near enough to the wire (~50 μm), an avalanche multiplication takes place with an amplification of 104 - 105 of the primary charge signal which allows the readout of the electric signal. The arrival time of the signal defines the drift radius, and the charge collected is proportional to the particle energy lost by ionization.
The PANDA Straw Tube Tracker consists of an ensemble of 150 cm long drift tubes, arranged in an cylindrical volume around the beam axis. The basic module is a planar double-layer of tubes in order to resolve the left-right ambiguity of the track position with respect to the wire.

STT internal radius

15cm

STT externalrnal radius

42cm

Number of double-layers

12

Skew angle of double-layer 5

+3o

Skew angle of double-layer 6

-3o

Tube wall thickness

30μm

Tube internal diam.

10mm

Axial tubes length

150cm

Wire diameter

20μm

Tube wall material

Al-mylar

Wire material

Au-plated W/Re

Gas mixture

Ar(90) - CO2(10)

Single tube transparency

3.7x10-4 X/X0

ρ/φ plane resolution

150μm

z resolution

1mm

Table 1: Straw Tube Tracker main parameters.

The first and the last four double-layers are parallel to the beam axis, while the intermidiate ones are arranged with a skew angle of ± 3o. This is necessary to reconstruct the particles z coordinate.
Among the tracking devices straw tubes show many advantages: